All right, I've done mod for my MK IV. MK III mod will be similar. Folks, this is a
must mod especially for MK IV SE users here in US, otherwise, you will run risk of burn out 6H30Pi-EH like mine does. MK III and MK IV runs power tube heater
unregulated AC straight from toroidal transformer!!! Lots of forum actually recommend 6.2V heater voltage for 6H30Pi, too high heater voltage will reduce its life drastically.
Unfortunately my burn-out 6H30Pi-EH did not recover at the lower heater voltage, it's gone for ever.
Here are the mod photos:
1. Pull out the board, the heater voltage line (white) are directly soldered to the 9-pin socket.
2. Solder a 1ohm 2W resistor in series. Mr. Yang recommend 1ohm 1W, but 6H30Pi heater eats ~800mA current, it's better to have some leeway for this resistor. Also Mr. Yang said matching is not necessary, but I matched those resistors anyway. Make sure you leave some space from the resistor to board, this resistor will run hot! And don't let it short anything, use heat shrink to protect all naked wires and solder joints.
3. After mod, test the heater voltage of the power tube, use a true RMS multimeter to measure AC voltage between pin 4 and pin 5. It used to be start 7.0V, and slowly drop to 6.8-6.9V. Now with this mod, it starts from ~5V, and slowly climb to 6.3-6.4V, because cold heater has smaller resistor than fully heated one, this mod also serves soft start functionality.
Picture here shows the heater voltage for regular Sovtek 6H30 after the mod, EH 6H30Pi is a little bit lower at 6.325V.